1. Field of the Invention
The present invention relates to a current measuring apparatus and a test apparatus as well as to a coaxial cable and an assembled cable used for the apparatuses. More specifically, the invention relates to a current measuring apparatus having coils that generate voltage representing current-under-measurement and to a low impedance coaxial cable and an assembled cable used for transmitting the current-under-measurement from a current supply.
2. Related Art
Conventionally, there has been known a current measuring apparatus for measuring electric current based on a magnetic field generated by the current-under-measurement. For example, a current probe measures electric current based on voltage generated in a secondary coil of a transformer corresponding to the current-under-measurement flowing through a primary coil thereof.
In case of measuring source current of an electronic device, a power terminal of the electronic device is connected with a current measuring apparatus by a coaxial cable or the like to input current-under-measurement to the current measuring apparatus. An observation zone is determined by such factors as capacity, inductance and characteristic impedance in such measurement.
Because the capacity of a supply line is unchangeable here, it is desirable to reduce the inductance and the characteristic impedance in order to widen the observation zone. As an example of such low impedance coaxial cable, there has been disclosed one in which a conductive laminate made of aluminum foil and others pasted on a film base material made of polyester or polyimide as its substrate is wound around an outer periphery of a conductor while putting the aluminum foil on the conductor side and an insulation layer is provided further around the outer periphery of the laminate (Japanese Patent No. 1992-56408).
However, it has been difficult to measure the current at high precision by the conventional current probe due to an influence of inductance of the line through which the current-under-measurement flows. Therefore, it has been also difficult to perform IDDT test at high precision in testing an electronic device for example. Still more, because the inductance increases in proportion to a square of a number of turns of the primary coil in the current probe, insertion impedance increases in measuring the current if the number of turns of the primary coil is 2 or more. Therefore, it has been troublesome to provide the current probe in which the number of turns of the primary coil is 2 or more and to measure micro-current at high precision by such current probe.
It has been also known that the closer an outer diameter of a signal line (core line) to an outer diameter of an insulator provided around the signal line, the smaller a value of characteristic impedance of a coaxial cable becomes. Here, the coaxial cable described above is arranged so as to bring an outer diameter of the signal line closer to an outer diameter of the insulator by increasing an effective sectional area and an effective radius of the conductor, i.e., the signal line, by means of a conductive laminator. However, the coaxial cable described above is intended for 50Ω or more of value of characteristic impedance and cut-through resistance is increased by fully increasing a thickness of the surrounding insulator.
Meanwhile, it is necessary to extremely thin the thickness of the insulator to relatively reduce the outer diameter of the insulator in order to realize a coaxial cable whose value of characteristic impedance is several Ω for the purpose of measuring source current of an electronic device. It is then necessary to facilitate uncovering of the insulator by increasing the cut-through resistance and to satisfy threshold voltage of the insulator in the same time in order to realize it.